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Since ambient reflections are removed and only very small quantities of power are needed due to the close contact between the transducer and the ear, headphones may offer a technically better listening experience. Transducers can be operated at a small percentage of their full excursion capabilities due to the low power need, which lowers THD and other non-linear distortions. Because it uses both transistors and valves in the same design and has a unity voltage gain, this particular headphone amplifier design may cause controversy. An impedance of 32R per channel is seen in typical headphones. A headphone with this impedance will provide a power of U2 / R = 0.7752 / 32 = 18 mW per channel when connected to the typical standard line output of 775 mV, to which all high-quality equipment aspires.
When headphones from well-known high street emporiums were examined, it was found that their sensitivity ranged from 96 dB to 103 dB/mW! Thus, in actual use, the circuit will only need unity gain to achieve loud levels. One can use a low distortion output stage since a unity gain design is necessary. The emitter follower is the obvious option.This has a lot of local feedback along with a gain of almost unity. Regretfully, the source impedance determines the emitter follower's output impedance. This will vary with a volume control or even with different signal sources, and it may result in subtle but noticeable changes to the sound quality. In order to avoid this, a cathode follower that is built around an ECC82 valve (US equivalent: 12AU7).
Unlike a transistor arrangement, this device allows low impedance, constant value driving of the output stage. Stated differently, low total THD is promoted because the output stage's high impedance is driven by the signal from the low impedance point. The only logical option for the circuit's moderate output powers is a Class A circuit. In this instance, the highly regarded single-ended output stage—which consists of T3 and constant current sources T1–T2—is used. The voltage across R5 of T1 (Vbe) determines the constant current. Its value of 22R sets the current to 27 mA. T3 is employed in high input emitter follower mode with high input impedance and low output impedance. It is true that the primary issue with employing a valve at low voltages is that it can be challenging to get any significant current drain.
The output stage shouldn't be permitted to load the valve in order to avoid distortion. This depends on the output device selection. Because of its large current gain of 30,000 at 2 mA, a BC517 is utilized for T3! We can use C4 to capacitively link the load because our output stage has a low impedance. Although some purists would object to the thought of employing an electrolytic for this purpose, the distortion produced by capacitive coupling is still at least two orders of magnitude less than that produced by transformer coupling. The circuit's varied voltages are conditioned by the remaining components.
The biasing of the valve grid must be at half the supply voltage in order to produce a linear output. This is how the voltage dividers R4 and R2 work. C1 and R1 link input signals into the circuit. The input impedance of the circuit is defined by R1, which is linked between the voltage divider and V1's grid. C1's value is large enough to guarantee response at frequencies as low as 2 Hz. Due to the high impedance of V1's anode and T3's collector current, the circuit performs a decent job of rejecting line noise on its own; nevertheless, it requires some assistance to provide a silent background when there is no signal. The circuit centered on T5 is a capacitance multiplier, which provides assistance. Here, a second BC517 is utilized to prevent the R7 and C5 filter from being loaded. In theory, the gain of T5 multiplies the capacitance of C5. In actuality, the emitter of T5 exhibits low impedance when smooth DC is given to its base.
The fact that the supply voltage is supplied gradually when the device first powers up is a significant bonus. Naturally, this is because it takes longer for C5 to charge fully via R7. Nothing audible on the scope, not even a hint of hum or ripple. To guarantee stability at RF, C2 is utilized. The valve heater is likewise powered by the DC supply. One feature of the ECC82 is that it can work on 12.6 V when connected as a heater. T4 is used as a series pass element in order to run it. T5's emitter provides the base voltage. Because of its extremely low output impedance of about 160 mR, the T4 helps shield the heater wiring from picking up unnecessary signals. The valve heater can also be made to gradually warm up by connecting the transistor base to C5. Only a few volts are lost over T4, and even if the gadget overheats, a heatsink is not necessary.